Hydroxynaphthyridine-Derived Group III Metal Chelates: Wide Band Gap and Deep Blue Analogues of Green Alq₃ (Tris(8-hydroxyquinolate)aluminum) and Their Versatile Applications for Organic Light-Emitting Diodes

Abstract

A series of group III metal chelates have been synthesized and characterized for the versatile application of organic light-emitting diodes (OLEDs). These metal chelates are based on 4-hydroxy-1,5-naphthyridine derivates as chelating ligands, and they are the blue version analogues of well-known green fluorophore Alq(3) (tris(8-hydroxyquinolinato)aluminum). These chelating ligands and their metal chelates were easily prepared with an improved synthetic method, and they were facially purified by a sublimation process, which enables the materials to be readily available in bulk quantity and facilitates their usage in OLEDs. Unlike most currently known blue analogues of Alq(3) or other deep blue materials, metal chelates of 4-hydroxy-1,5-naphthyridine exhibit very deep blue fluorescence, wide band gap energy, high charge carrier mobility, and superior thermal stability. Using a vacuum-thermal-deposition process in the fabrication of OLEDs, we have successfully demonstrated that the application of these unusual hydroxynaphthyridine metal chelates can be very versatile and effective. First, we have solved or alleviated the problem of exciplex formation that took place between the hole-transporting layer and hydroxynaphthyridine metal chelates, of which OLED application has been prohibited to date. Second, these deep blue materials can play various roles in OLED application. They can be a highly efficient nondopant deep blue emitter: maximum external quantum efficiency eta(ext) of 4.2%; Commision Internationale de L'Eclairage x, y coordinates, CIE(x,y) = 0.15, 0.07. Compared with Alq(3), Bebq(2) (beryllium bis(benzoquinolin-10-olate)), or TPBI (2,2',2''-(1,3,5-phenylene)tris(1-phenyl-1H-benzimidazole), they are a good electron-transporting material: low HOMO energy level of 6.4-6.5 eV and not so high LUMO energy level of 3.0-3.3 eV. They can be ambipolar and possess a high electron mobility of 10(-4) cm(2)/V s at an electric field of 6.4 x 10(5) V/cm. They are a qualified wide band gap host material for efficient blue perylene (CIE(x,y) = 0.14, 0.17 and maximum eta(ext) 3.8%) or deep blue 9,10-diphenylanthracene (CIE(x,y) = 0.15, 0.06 and maximum eta(ext) 2.8%). For solid state lighting application, they are desirable as a host material for yellow dopant (rubrene) in achieving high efficiency (eta(ext) 4.3% and eta(P) 8.7 lm/W at an electroluminance of 100 cd/m(2) or eta(ext) 3.9% and eta(P) 5.1 lm/W at an electroluminance of 1000 cd/m(2)) white electroluminescence (CIE(x,y) = 0.30, 0.35).

Push-pull effect on the geometrical, optical and charge transfer properties of disubstituted derivatives of mer-tris(4-hydroxy-1,5-naphthyridinato) aluminum (mer-AlND3)

“…We have also explained that the basis set has no significant effect on absorption wavelength and the absorption and emission energies have been calculated by TDDFT at the PBE0/6-31G(D) level using B3LYP/6-31G(D) and CIS/6-31G(D) optimized geometries respectively. Here also similar to the previous study the absorption and emission wavelengthsof (1)(2)(3)(4)(5)(6)(7)(8) have been computed at the same level of theory and are summarized in Table 6, along with the experimentally reported absorption and emission wavelengths of merAlND3 [52] The ligand atom labels used in the text for the disubstituted AlND3 complexes considered in this study in Table 4 and Table S2(a-d) (for EDG and EWG labels are not shown). The NBO charges (Q) of the ligands in 1 and 5, in the optimized ground (Q HF-S0 ) and first excited (Q S1 ) states structures and the charge differences between Q HF-S0 and Q S1 states calculated at the HF/6-31G(D) and CIS/6-31G(D) methods, respectively (for atom labels see Fig.…”

“…The geometry of these derivatives is constructed by using the reported x-ray coordinates of the parent mer-AlND3 [52], which possesses C 1 -symmetry. The geometry optimizations have been carried out using DFT/B3LYP/6-31G(D) method using G09 package [53].…”

“…The model disubstituted derivatives (1)(2)(3)(4)(5)(6)(7)(8) used in this study are obtained by systematic substitution of EDG (-CH 3 /NH 2 ) and EWG (-CN/Cl) at 8 and 2-positions respectively on each ligands of mer-AlND3 as shown in Fig.1, considering the reported parent mer-AlND3 crystal geometry [52]. The ligands are labeled with A-C designating the three naphthyridinato ligands substituted with different aryl EWG/EDGs in the 8 and 2-positions as shown in Fig.1.…”

“…The calculated geometrical parameters of (1)(2)(3)(4)(5)(6)(7)(8) are shown in Table 1, along with the optimized parameters of merAlND3 and the experimental [52] parameters for the same are given for comparison. Variation in the calculated bond lengths and bond angles have been observed for all the disubstituted derivatives, when compared to calculated geometrical parameters of parent molecule (mer-AlND3).…”

“…The electron withdrawing groups such as chloro [39] and cyano [16] show almost negligible emission shifts, while the strong electron withdrawing group such as sulfonamide results in blue shifted emission [50] and the substitution of fluorine at different positions in mer-Alq3 ligand show variable emission [51]. The mer-AlND3 and its methyl derivatives are reported to be the blue version analogues of mer-Alq3 and can be used as electron transporting layer as well as emitting layer in the OLEDs [52]. So in the present study our main aim is to design theoretically some new novel emitting materials with tunable color as well as high charge mobility by introducing electron donating groups (EDG) and electron withdrawing groups (EWG) at 8 and 2 positions and vice versa on 4-hydroxy-1,5-naphthyridinato ligands respectively in mer-AlND3.…”